This week, three women are graduating from the Marine infantry training course. They’re the first female candidates to pass it. The list of things women supposedly can’t do is evaporating.

This is what happens when you stop excluding whole groups of people from tests and careers. Some of them want those careers. Some of them can pass those tests. How many? You don’t know. In sports parlance, that’s why you play the game. You open the competition and let it surprise you. Not everything turns out equal. But sooner or later, you get a Jewish swimming prodigy, a gay diving champ, a black golfing legend or a Chinese basketball star.

Until this year, no woman had attempted the infantry course. Girls weren’t allowed. Under the military’s Direct Ground Combat Definition and Assignment Rule, women were “excluded from assignment to units below the brigade level whose primary mission is to engage in direct combat on the ground.” In January, Defense Secretary Leon Panetta rescinded that policy. The new policy seeks “the best-qualified and most capable people, regardless of gender.”

Critics said the military would have to lower its standards to let women make the cut. But the Marines held firm. Their infantry course is two months long. You sleep in holes. You march 12 miles wearing 85 pounds of gear. You run 6 miles. You learn marksmanship, martial arts and urban combat.

In September, 114 women graduated from the Marine boot camp at Parris Island, S.C. At every stage, their numbers dwindled. Forty-two passed the physical requirements for training. Nineteen volunteered to do it. Fifteen followed through. Seven made it through the first month. Three graduated.

That’s a lot lower than the male success rate. Why? Some of the difference is cultural, but much of it is physical. On average, women have less upper body strength than men do. Under heavy burdens, they’re more prone to stress fractures. But these are averages. Many men can’t hack it. Some women can.

The three women who graduate this week are just the beginning. Forty more have entered the pipeline. The next, more strenuous challenge is the Marine Infantry Officer Course. So far, 10 women have attempted it. Only one passed the hardest part, the Combat Endurance Test, and a stress fracture later forced her out. Once that barrier falls, the Army and Navy will have to open their own ground combat training to female candidates.

The old guard is already falling back on other excuses. Rep. Duncan Hunter, R-Calif., a former Marine whose father chaired the House Armed Services Committee, thinks the paucity of women attempting the Marine officer course shows they don’t really want the job. He tells The Washington Post, “If you only have 10 women who are interested, then what is the uproar all about?”

That’s hogwash. One reason why so few women apply is that they’re denied the incentive given to men: Even if the female candidates pass, the Marines won’t let them earn an infantry specialty. The broader level of interest among women has surprised Marine officials. In a survey last year, 34 percent of female Marines said they’d volunteer to serve in a ground combat unit. At Parris Island, 51 percent said they’d consider infantry training.

What’s keeping women out of the infantry altogether isn’t the weakness of women. It’s the weakness of men. In last year’s survey, 17 percent of male Marines said they’d probably leave if women were allowed in combat jobs. Commanders are afraid to lose those men. The few, the proud, the insecure.

A century and a half ago, Theodore Parker, the great abolitionist, declared that the arc of the moral universe bends toward justice. Sometimes that arc leads to equality of outcome. In the case of gender and physical strength, it doesn’t. Women, on average, will always be weaker than men at climbing a rope or carrying a pack. But some women can make it to the top. Some can finish the course. If we substitute merit for prejudice, we can’t be sure where the universe will take us. But it will certainly take us away from group-based exclusions.